In order for an electroless metal plating bath to plate the desired metal with good physical properties, e.g., ductility, adhesion, density, etc., at an acceptable deposition rate, a number of chemical components of the plating solution must be controlled within specific ranges. For example, the major components that are normally analyzed in an electroless copper plating solution are copper, sodium hydroxide and formaldehyde. The maintenance of the concentrations of these components within specified ranges is critical to the proper operation of the electroless plating bath and the quality of the resultant metal film deposited therefrom. Traditionally, the quantitative analysis of these constituents has been performed off-line, in a process control laboratory. Based upon the results of these off-line analyses, the quantities of make-up to maintain the bath within proper operating ranges was calculated and the appropriate quantity was added to the plating bath. This method was not only time consuming, but due to the time lag and inaccuracy found in prior art analytical techniques, various other problems existed.
In recent years, some commercial control units have become available for on-line use, however, these controllers do not provide quantitative information. They are merely setpoint controllers as opposed to an analyzer-controller combination. Setpoint controllers respond to deviations from a desired level but give no information about the actual concentration of the component being controlled. Anaylzer-controllers, on the other hand, provide information about the component concentration as well as perform the control function.
We have now invented an automated, computer controlled method for analyzing and controlling the bath chemistry of electroless metal plating baths which allow unattended operation of the bath for extended periods of time, as well as the ability to monitor the bath chemistry.